This invention generally relates to a flexure based linear guide bearing. Flexures have been used successfully in motion systems for centuries. Since they operate by bending, not rolling or sliding, flexures have the inherent advantage of friction-free motion. This key feature allows engineers to build positioning systems with nearly unlimited precision and accuracy. They are also easily fabricated using readily available materials and common methods. However, even with these desirable attributes there are very few flexure based linear bearings available commercially. Therefore, it is common practice for an engineer to design custom flexure systems when developing a new mechanism. The linear flexure bearing described herein is particularly well suited for integration into precision motion systems and opto-mechanical mechanisms where friction-free motion over a limited range is required. It also has a configuration that scales easily without compromising the operating principle, making this concept a convenient basis for a family of linear flexure bearings. In addition to applications in precision mechanics, this bearing can be used in hostile operating conditions (extreme temperatures, extreme changes in temperature, vacuum, corrosive environment, contaminated environment . . . ) that normally prohibit use of conventional bushings, rolling element bearings, or gas lubricated bearings.
The current state of technology for flexure translation bearings is based on parallel planar flexures. A simple parallel flexure translation bearing at mid travel is shown in FIG. 2. It is composed of a fixed member 10 and an active member 20 connected by a pair of flexure members 30. When the active member is displaced in the + or −X direction it also moves in the −Y direction due to foreshortening of the flexure members when they bend as shown in FIG. 3.
A compound linear flexure bearing is formed when two simple parallel flexure translation bearings are assembled in series as shown in FIG. 4. One has the active member 40 and the other has the fixed member 50. They are connected by a common intermediate member 60. When a load is applied to the active member, the load path is from the active member through the first pair of flexure members, through the intermediate member, then through the second pair of flexure members and into the fixed member. If all four flexure members have the same stiffness and share the same dimensions, then they will experience the same Y foreshortening. The X deflection of each flexure member will be the same as the intermediate member which is half the active member X displacement. The rate of change and amount of Y foreshortening of the flexure member pair between the fixed member and intermediate member is identical to that of the flexure member pair between the intermediate member and active member. Since this flexure member Y foreshortening causes the intermediate member to move closer to the fixed member and the active member closer to the intermediate member, the net Y displacement of the active member is zero as shown in FIG. 5. It is desirable for the active member to maintain a constant Y position at all X positions, and this arrangement of two simple parallel flexure translation bearings makes it possible. However, the intermediate member is constrained in the X direction only by the stiffness of the flexure members. This means inertial loading, shock and vibrations, as well as active member accelerations during normal operation could excite the intermediate member causing it to oscillate after the active member has come to rest. FIG. 6 shows the intermediate member displaced while the active member is fixed at mid travel. The two simple parallel flexure translation bearings are shown operating in parallel rather than in series. Since the two simple parallel flexure translation bearings are allowed to simultaneously function in series and parallel, the location of the intermediate member can be difficult to determine. If the intermediate member oscillates on the flexure members after the active member has been moved to the desired position, the system may have stability issues.
The performance of a compound linear flexure bearing can be improved by regulating the position of the intermediate member with respect to the fixed and active ends. A lever arm 70 that is connected to a flexure pivot 80 on the fixed side and attached to both the active and intermediate members by flexure members 90 is typically used to control the intermediate member position as shown in FIG. 7. Since the lever arm positively locates the intermediate member, the two simple parallel flexure translation bearings will only operate in series as shown in FIG. 8. This version of the compound linear flexure bearing settles quickly and is not easily excited by external shock or vibration. Adding a lever arm to the compound linear flexure bearing improves stability, but comes with some tradeoffs. One is that the lever arm must be twice the length of the flexure members in the sub-stages so that the active member has a 2:1 mechanical advantage over the intermediate member. Accommodating this lever arm and its related hardware may present a packaging challenge when integrating the stage into a system. Other concerns are extra mass, mechanical complexity, and additional expense.